Blow through direct fired heating, A/C and ERV

ABSTRACT

According to various aspects, exemplary embodiments are disclosed of blow through direct fired heaters including evaporator coils and/or energy recovery ventilation.

FIELD

The present disclosure generally relates to blow through direct firedheaters, which may include air conditioning and/or energy recoveryventilation.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Direct gas-fired heaters have been manufactured for over 50 years toserve industrial and commercial facilities. In direct fired commercialheaters, circulation air and products of combustion are vented directlyinto the space being heated, unlike indirect fired heaters that ventcombustion products to the outdoors. Direct gas-fired heaters areprimarily intended for space heating applications in commercial andindustrial facilities to address the heat load and ventilationrequirements of these facilities.

Direct gas-fired heaters have also been marketed for over 50 years witha blow-through heater configuration in which the blower is upstream ofthe burner. More specifically, the blower is located to handle outsideair and blow the outside air past a burner, which is operable forheating the outside air before it is discharged into the space to beheated.

Direct fired blow-through heater configurations are well suited for useas space heaters. In this case, a direct blow-through heater may beapplied to address the heat load of a facility and not to match a givenexhaust application. Industrial and commercial buildings have aninfiltration load element as part of its heat load as a result of windand temperature differences between indoor and outdoor temperatures.Based on ASHRAE (American Society of Heating, Refrigeration, andAir-Conditioning Engineers) ventilation requirements, it is oftennecessary to provide a source for this ventilation requirement as wellas which can be met by this same heater.

In some well insulated buildings, the infiltration element of the heatload analysis can show that the infiltration load and the loadassociated with the ventilation requirement are more significant thanthe conduction load. In these applications, the optimization of aheating system occurs when the system first addresses and matches thecombination of infiltration load and ventilation load on a designatedday and then checks to verify that the conduction load requirement hasalso been addressed. When a direct fired heater is utilized for spaceheating, that portion of the heater's capacity that heats the outsideair temperature to room temperature is directly tied to the infiltrationand ventilation heat load. That portion of the heater capacity aboveroom temperature and the maximum temperature rise of the heater areapplied to the conduction load with any extra capacity also beingapplied to any infiltration and ventilation heat load remaining, ifrequired. There is a significant system efficiency advantage if theblow-through heater is capable of obtaining a temperature rise equal tothe maximum discharge temperature allowed by the ANSI (American NationalStandards Institute) Standard Z83.4 for Non-Recirculating DirectGas-Fired Industrial Air Heaters. ANSI Standard Z83.4 sets the maximumdischarge temperature at 160° F. and limits the maximum temperature riseto 190° F. In an application where the minimum design for a location is0° F. (e.g., like Saint Louis, Mo., etc.), a heater with a temperaturerise of 160° F. would therefore optimize the heater selection for thatlocation.

Another benefit of a direct fired blow-through space heaterconfiguration is that a space heater is generally cycled on and offbased on a call for heat by a room thermostat. A conventionaldraw-through make-up air heater will run continuously as long as theexhaust fan is operating. During the operating time of a space heater,the heater airflow tends to neutralize the flow of infiltration air intothe building as a result of the air brought in by the heater escapingout of the same cracks. This exhale of the air supplied by the heatercarries out other contaminants that may be created in the building. Ifthe infiltration rate of the building is too low, additional reliefopenings may be required to meet the minimum ventilation requirements ofthe facility.

Air conditioning may be used to alter the properties of air to morefavorable conditions, typically with the aim of distributing theconditioned air to an occupied space to improve comfort. Commonly, airconditioning lowers the air temperature through cooling, although otherconditioning effects may also be implemented. The cooling is typicallydone using a refrigeration cycle (sometimes including direct expansioncoiling), but other suitable technologies may also be used.

Energy recovery ventilation is the energy recovery process of exchangingenergy contained in normally exhausted building or air space air andusing it to treat (precondition) the incoming outdoor ventilation air inresidential and commercial heating, ventilation and air conditioning(HVAC) systems. During the warmer seasons the system pre-cools anddehumidifies, and during the cooler seasons the system pre-heats andhumidifies.

The inventors have recognized that a combination of blow through directfired heating systems, direct expansion cooling, and energy recoveryventilation may allow for increased energy efficiency, as well asimproving indoor air quality. The inventors have also recognized thatcombining the evaporator coil of an air conditioning system with the airflow of a direct fired system can lead to formation of harmful productsof combustion (e.g., phosgene gas) due to the possible combustion ofrefrigerants as they pass the flame of a direct fired system.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

According to various aspects, exemplary embodiments are disclosed ofblow through direct fired heaters including evaporator coils and/orenergy recovery ventilation.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure

FIG. 1 is a perspective view of a blow through direct fired HVAC systemwith an evaporator coil upstream of the burner according to exemplaryembodiments;

FIG. 2 is another perspective view of the blow through direct fired HVACsystem shown in FIG. 1;

FIG. 3 is a side view of the blow through direct fired HVAC system shownin FIG. 1;

FIG. 4 is a perspective view of a blow through direct fired HVAC systemwith an evaporator coil downstream of the burner according to exemplaryembodiments;

FIG. 5 is another perspective view of the blow through direct fired HVACsystem shown in FIG. 4;

FIG. 6 is a side view of the blow through direct fired HVAC system shownin FIG. 4;

FIG. 7 is a perspective view of a blow through direct fired HVAC systemwith energy recovery ventilation according to exemplary embodiments;

FIG. 8 is another perspective view of the blow through direct fired HVACsystem shown in FIG. 7; and

FIG. 9 is a side view of the blow through direct fired HVAC system shownin FIG. 7.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

The inventors have recognized that combining a blow through direct firedheating system with direct expansion coiling can provide energyefficiency as well as improving indoor air quality, but that combiningthe evaporator coil of an air conditioning system with the air flow of adirect fired system can lead to formation of harmful products ofcombustion (e.g., phosgene gas) due to the possible combustion ofrefrigerants as they pass the flame of a direct fired system.

By way of example, an exemplary embodiment of a direct fired heatingsystem having direct expansion cooling with an evaporator coil upstreamof a burner generally includes an air switching box and a safetycircuit. When the system is in a cooling and/or air conditioning (A/C)mode, air is either returned from the indoor space, brought in from theoutside, or a combination of the two. The safety circuit and airswitching box ensure that the burner is off prior to any air beingcirculated across the coil and distributed into the environment.

When the system is in a heating mode, air is supplied directly fromoutdoors. The safety circuit ensures that when the burner is on, theoutdoor supply of air is directed so as to not pass through theevaporator coil prior to entering the burner. This ensures any potentialleak of refrigerant cannot pass through an open flame.

In another exemplary embodiment, a blow through direct fired heatingsystem having direct expansion cooling with an evaporator coildownstream of a burner generally includes an air switching box and asafety circuit. Because the coil is oriented downstream of the burner'sflame, no generation of harmful phosgene gas will occur. The airswitching box and safety circuit will only divert air across theevaporator coil when in cooling or A/C mode. The air switching box willdivert air so as to not pass through the coil when the system is inheating mode.

Diverting air around the evaporator coil during heating mode isimportant because exposure of high temperatures (e.g., 160 degreeFahrenheit air) across an evaporator coil would generate excessivepressures in the coil. This could cause unsafe working conditions, orrequire installation of an extremely robust coil that would not beeconomical to produce.

Adding one or more air conditioning components (e.g., an evaporatorcoil, compressor, expansion valve, condensing coil, condensing fans, A/Ccontrol, etc.) to a blow through direct fired gas heating system mayallow for efficient gas heating and cooling in a compact, lightweight,easy to install, packaged HVAC system.

Due to the high static capability and high discharge velocity of such asystem, it may be used in a number of different applications. Forexample, it can be installed in large open areas with the use of highvelocity discharge elbows to mitigate stratification. Alternatively, itcan be used in small separated areas with the addition of dischargeductwork to supply comfort ventilation to residential and commercialbuildings. The combination of these technologies (i.e., blow throughdirect fired heating combined with air conditioning) in one package maysignificantly reduce the overall installation cost, as compared toinstalling separate heating and cooling systems.

Outdoor air is sometimes used in HVAC systems to improve indoor airquality. Building codes across the United States and other countries arestarting to adopt and mandate indoor air quality standards. According toanother exemplary embodiment, the combination of blow through directfired technology, variable outside air A/C systems, and energy recoveryventilation (ERV) can further increase both the efficiency and indoorair quality of an HVAC system. Due to the nature of improving indoor airquality with the use of direct fired air and an outdoor air supply of anA/C system, a building may slightly pressurize, requiring conditionedair to be pushed out of a building. The combination of an ERV with theHVAC system can recover approximately 50-70% of the energy lost andreintroduce it to the incoming air stream.

The HVAC system may be optimized using air quality control. According tosome example aspects of the present disclosure, monitoring of carbondioxide (CO₂), humidity and/or temperature (e.g., indoor, outdoor, drybulb, etc.) may be correlated to an optimal air flow, gas modulationand/or outside versus inside air percentage combination to increase theefficiency and quality of the air combination.

When packaging a system with blow through direct fired heatingtechnology with A/C and/or ERV technology, each system may operate mosteffectively under different amounts of air flow. In order to optimizethe combined systems using one source of air, variable air deliverysystem logic is combined with a variable mixing of inside to outsidesupplied air according to another example aspect of the presentdisclosure.

According to this example aspect, the supply of air will be delivered atdifferent flow rates and/or combinations of inside to outside airpercentages based on one or more system parameters, including withoutlimitation, inside temperature, outside temperature, dry bulbtemperature, wet bulb temperature, indoor air quality requirementsascertained by CO₂ detection, etc. The system may include a variablefrequency drive (VFD) for adjusting the fan speed of a blower, variabledampers for adjusting the mixture of outside and inside air, and acontroller for controlling the VFD and variable dampers based on the oneor more sensed system parameters.

With reference now to the figures, FIGS. 1 through 3 illustrate anexemplary embodiment of a blow through direct fired HVAC system 100embodying one or more of the various aspects and features disclosedherein. As shown in FIGS. 1-3, a burner 102 is configured to heat aircirculated across an open flame of the burner 102. The burner 102 may beany suitable burner, such as, for example, a gas burner supplied by autility gas line with a gas that is ignited to create an open flame atthe burner 102. Although only a single burner 102 is illustrated inFIGS. 1-3, other embodiments may include more than one burner 102.

A blower 104 (e.g., a supply air blower) is disposed upstream of theburner 102 and configured to circulate air across the open flame of theburner 102. Accordingly, the system 100 in this example has a directfired blow through heater configuration when operating in a heatingmode. The blower 104 is driven by motor 106. The blower 104 may be anysuitable blower, such as, for example, a primary circulating air blower,and may include one or more fans. Although only a single blower 104 andsingle motor 106 are illustrated in FIGS. 1-3, other embodiments mayinclude more than one blower 104 and/or more than one motor 106.

The system 100 may also be configured to operate in an air conditioningmode. An evaporator coil 108 is disposed upstream of burner 102. Theevaporator coil 108 may be any suitable type of evaporator coil, suchas, for example, an evaporator coil configured to cool air throughdirect expansion. Although only one evaporator coil 108 is illustratedin FIGS. 1-3, other embodiments may include more than one evaporatorcoil 108.

Other components in the system 100 operable during an air conditioningmode include compressor 110, condensing coil 112, condensing coil fan114, an expansion valve (not shown), and a controller (not shown). Thecompressor 110 is configured to pressurize a refrigerant. The condensingcoil 112 is disposed outside for contact with outside air. Thecondensing coil 112 is coupled to the compressor 110 and configured toreceive the pressurized refrigerant from the compressor 110 and convertthe refrigerant from a gas to a liquid. The condensing coil fan 114 isconfigured to circulate air across the condensing coil 112. An expansionvalve is coupled to the condensing coil 112 and configured to removepressure from the refrigerant before providing the refrigerant to theevaporator coil 108. A controller is coupled to one or more of theevaporator coil 108, the compressor 110, the condensing coil 112, thecondensing coil fan 114, and the expansion valve. The controller isconfigured to operate system 100 in a cooling and/or air conditioningmode. Although one example configuration for providing cooling and/orair conditioning is illustrated in FIGS. 1-3, any other suitable coolingconfigurations could be used instead, including more, less, or differentair conditioning components without departing from the scope of thepresent disclosure.

The system 100 also includes components for allowing air to enter andcirculate through the system 100. A first inlet includes a first damper116 configured to allow outside air to enter the system 100 when opened.The outside air passes through filter 118 and inlet screen 120, whichare coupled to rain hood 122. Rain hood 122 prevents rain water fromentering the system 100, while filter 118 and inlet screen 120 preventunwanted particles in the outside air from entering the system 100.

A second inlet includes a second damper 124 and a third damper 126. Whenthe third damper 126 is open, outside air may enter the system 100, andpass through filter 128 and evaporator coil 108, thereby cooling theair. The second damper 124 allows the air to pass through to the blower104 when open. The second damper 124 may also allow recirculated insideair to pass through the damper 124 when open, and therefore may combinepreviously cooled inside air with outside air received from third damper126 and cooled by evaporator coil 108. Although the system isillustrated with a rain hood 122, filters 118 and 128, and inlet screen120, other embodiments may include more, less, or different componentsfor filtering and/or screening unwanted particles and/or water fromentering the system 100.

The three dampers 116, 124, and 126 may be considered as an air mixingsection, or an air switching box, of the system 100. When the system 100is operating in a heating mode, the first damper 116 is opened and thesecond and third dampers 124, 126 are closed. This allows 100% outsideair to pass through the blower 104 and burner 102 of the direct firedblow through heating configuration. This prevents or inhibitsrecirculation of internal air, which is a requirement of ANSI Z83.4 forsafe operation of direct fired systems.

There may be micro switches on each damper 116, 124, and 126. A safetycircuit and a flame safety relay may ensure that the first damper 116 isopen and the second and third dampers 124, 126 are closed prior to theflame safety relay allowing the burner 102 to begin an ignition cycle.The dampers and safety circuit ensure that no air will pass over theevaporator coil 108 during heating operation, thereby preventing orinhibiting formation of harmful products of combustion (e.g., phosgenegas) due to the possible combustion of refrigerants as they pass theflame of a direct fired system.

When the system is operating in a cooling mode, the first damper 116 isclosed, while the second and third dampers 124, 126 are opened. Thethird damper 126 allows outside air to enter the cabinet. The seconddamper 124 allows mixed recirculated indoor air and outside air fromdamper 126 to enter the upper portion of the cabinet of system 100. Whenthe second and third dampers 124, 126 are fully opened, they provide a50%/50% split of outside and recirculated inside air to flow through theevaporator coil 108. This provides outside fresh air to a building tomeet ventilation requirements and recirculated indoor air to be moreeconomical, because cooling 100% outside air requires significantly moreenergy.

The safety circuit ensures that the first damper 116 is closed and thatthe second and third dampers 124, 126 are opened prior to the airconditioning compressor 110 and the fan motor 106 being turned on. Thesafety circuit will also ensure the flame safety relay will not energizethe burner combustion system. This ensures that in the event of anevaporator coil refrigerant leak, no refrigerant will pass over the openflame of the burner causing harmful products of combustion.

The system 100 may operate in a mode of 100% outside air for heating,and a 50%/50% outside/inside air mode for cooling. The system 100 mayalso operate in an economizer mode. When outside air conditions aremoderate enough to meet the requirements of the indoor thermostat, thesystem 100 may provide an economizer mode. The dampers in the economizermode will operate in the same manner as the cooling mode, except theburner 102 and the air conditioning compressor 110 will not be turnedon. A mixture of outside and inside air will be brought through thesystem 100 and used to supply air to the indoor environment.

FIGS. 4 through 6 illustrate another example embodiment of a directfired blow through HVAC system 200 embodying one or more of the variousaspects and features disclosed herein. The example embodimentillustrated in FIGS. 4-6 is similar in some aspects to the exampleembodiment illustrated in FIGS. 1-3, but differs in some aspects aswell, including the evaporator coil 208 being positioned downstream ofthe burner 202.

As shown in FIGS. 4-6, a burner 202 is configured to heat air circulatedacross an open flame of the burner 202. A blower 204 is disposedupstream of the burner 202 and configured to circulate air across theopen flame of the burner 202. Accordingly, the system 200 in thisexample has a direct fired blow through heater configuration whenoperating in a heating mode. The blower 204 is driven by motor 206.

The system 200 may also be configured to operate in an air conditioningmode. As such, an evaporator coil 208 is disposed downstream of burner202. Other components in the system 200 operable during an airconditioning mode include compressor 210, condensing coil 212,condensing coil fan 214, an expansion valve (not shown), and acontroller (not shown).

The system 200 also includes components for allowing air to enter andcirculate through the system 200. A first inlet includes a first damper216 configured to allow outside air to enter the system 200 when opened.The outside air passes through inlet screen 220, which is coupled torain hood 222. A second inlet includes a second damper 224. When thesecond damper 224 is open, recirculated air from a building may enterthe system 200 through building air inlet 230.

The system 200 also includes a discharge mode damper 232, which may beconsidered an air switching box. The discharge mode damper 232 ismoveable between a first diverting position (shown in FIGS. 4-6) inwhich the discharge mode damper 232 causes air to pass over theevaporator coil 208 before being discharged from the system 200, and asecond blocking position (not shown) in which the discharge mode damper232 prevents or inhibits air from passing over the evaporator coil 208before being discharged.

There may be micro switches on each damper 216, 224, and 232. A safetycircuit and a flame safety relay may ensure that the discharge modedamper 232 is in an evaporator coil blocking position prior to the flamesafety relay allowing the burner 202 to begin an ignition cycle. Thedischarge mode damper 232 and safety circuit ensure that no air (or verylittle air) will pass over the evaporator coil 208 during heatingoperation. Although the evaporator coil 208 is downstream of the burner202 in this example embodiment such that refrigerants will not pass anopen flame of the burner, the discharge mode damper 232 ensures that theheated air does not pass over the evaporator coil 208, therebypreventing exposure of high temperatures (e.g., 160 degree Fahrenheitair) across the evaporator coil 208 that would generate excessivepressures in the coil. This could cause unsafe working conditions, orrequire installation of an extremely robust coil that would not beeconomical to produce.

When the system 200 is operating in a heating mode, the first damper 216is open and the second damper 224 is closed. This allows 100% outsideair to pass through the blower 204 and burner 202 of the direct firedblow through heating configuration. This prevents or inhibitsrecirculation of internal air, which is a requirement of ANSI Z83.4 forsafe operation of direct fired systems. The system 200 also ensures thedischarge mode damper 232 is in an evaporator coil blocking position toprevent exposure of the evaporator coil 208 to high temperatures.

When the system 200 is operating in a cooling mode, both the firstdamper 216 and the second damper 224 may be opened. The discharge modedamper 232 is moved to a diverting position to cause the air to passover the evaporator coil 208 before the air is discharged from thesystem 200. When the first and second dampers 216, 224 are fully opened,they provide a 50%/50% split of outside air and recirculated inside airto flow through the evaporator coil 208. This provides outside fresh airto a building to meet ventilation requirements and recirculated indoorair to be more economical, because cooling 100% outside air requiressignificantly more energy.

FIGS. 7 through 9 illustrate another example embodiment of a directfired blow through HVAC system 300 embodying one or more of the variousaspects and features disclosed herein. The example embodimentillustrated in FIGS. 7-9 is similar in some aspects to the exampleembodiment illustrated in FIGS. 4-6, but differs in some aspects aswell, including the energy recovery ventilation module.

As shown in FIGS. 7-9, a burner 302 is configured to heat air circulatedacross an open flame of the burner 302. A blower 304 is disposedupstream of the burner 302 and configured to circulate air across theopen flame of the burner 302. Accordingly, the system 300 in thisexample has a direct fired blow through heater configuration whenoperating in a heating mode. The blower 304 is driven by motor 306.

The system 300 may also be configured to operate in an air conditioningmode. As such, an evaporator coil 308 is disposed downstream of burner302. Other components in the system 300 operable during an airconditioning mode include compressor 310, condensing coil 312,condensing coil fan 314, an expansion valve (not shown), and acontroller (not shown).

The system 300 also includes components for allowing air to enter andcirculate through the system 300, as well as exit a building and beexhausted to the outside air. An inlet includes outside air passingthrough inlet screen 320, which is coupled to rain hood 322. Separately,an exhaust fan 336 blows exhaust air from inside a building, out of thesystem 300 through exhaust rain hood 338.

The system 300 also includes a discharge mode damper 332, which may beconsidered an air switching box. The discharge mode damper 332 mayinclude at least two diverter dampers. The discharge mode damper 332 ismoveable between a first diverting position (shown in FIGS. 7-9) inwhich the discharge mode damper 332 causes air to pass over theevaporator coil 308 before being discharged from the system 300, and asecond blocking position (not shown) in which the discharge mode damper232 prevents or inhibits air from passing over the evaporator coil 308before being discharged.

A safety circuit and a flame safety relay may ensure that the dischargemode damper 332 is in an evaporator coil blocking position prior to theflame safety relay allowing the burner 302 to begin an ignition cycle.The discharge mode damper 332 and safety circuit ensure that no air (orvery little air) will pass over the evaporator coil 308 during heatingoperation. Although the evaporator coil 308 is downstream of the burner302 in this example embodiment such that refrigerants will not pass anopen flame of the burner, the discharge mode damper 332 ensures theheated air does not pass over the evaporator coil 308, therebypreventing exposure of high temperatures (e.g., 160 degree Fahrenheitair) across the evaporator coil 308 that would generate excessivepressures in the coil. This could cause unsafe working conditions orrequire installation of an extremely robust coil that would not beeconomical to produce.

The system 300 also includes an energy recovery ventilation (ERV) device334. The ERV device 334 allows outside air to flow into the system 300in a first set of one or more channels, while also allowing inside airto be exhausted from inside the building through a second set of one ormore channels. The first and second sets of channels may be disposedadjacent to one another in perpendicular directions. In heating,cooling, and economizer modes, air may be exhausted through the ERVdevice 334 and released outdoors. As air passes through the ERV device334, sensible and latent energy transfer may occur between the incomingand exiting air streams. For example, the channels for the inlet air andthe exhaust air may be disposed in sufficient proximity (e.g., inthermal contact) such that energy may transfer between the inlet air andthe exhaust air. Although a block style ERV device 334 is illustrated inFIGS. 7-9, other suitable ERV devices may be used in other embodiments,such as, for example, ERV wheels capable of providing sensible andlatent heat transfer.

In colder outdoor conditions (e.g., winter), warm moist air is exhaustedand a portion of the energy (e.g., sensible and latent) is transferredto the incoming air. This preheated air then enters the blower 304 andburner 302. Depending on the temperature of the preheated incoming air,the burner 302 may turn on and modulate to meet the requirements of athermostat. The discharge mode damper 332 allows a straight path for airflow down into the building in heating or economizer modes. The airswitching box includes micro switches and a safety circuit that willensure the hot air from the burner 302 will not pass over the evaporatorcoil 308. The excessive temperatures of the hot air will be safelydiverted around the evaporator coil 308, preventing generation ofexcessive temperatures in the refrigerant of the evaporator coil 308,which could damage the system 300 and lead to refrigerant leaks andpossible harmful products of combustion.

In hotter outdoor conditions (e.g., summer), cold air is exhausted fromthe building through the ERV device 334. The incoming air will cool asit passes the ERV device 334. This precooled air will enter the blower304 and be delivered to the lower section of the system 300. Here theswitching box will divert the air over the evaporator coil 308. The airwill be further cooled by the evaporator coil 308 and will be deliveredinto the building to meet the thermostat requirements. The air switchingbox, micro switches, and a safety circuit will ensure that when thethermostat is in a cooling mode, the airflow is directed properly acrossthe evaporator coil 308, and the burner 302 is turned off.

When outside conditions are acceptable to meet the internal thermostatrequirements, the system 300 will go into an economizer mode with theA/C compressor 310 turned off and air diverted around the evaporatorcoil 308 directly into the building.

At least some of the example embodiments described herein may include avariable frequency drive (VFD) to control the rpm (revolutions perminute) of the fan motor upstream of the burner. This provides acapability to vary the amount of air delivered through the system and toadjust the varying static of two different air flows.

In a heating mode, the airstream may require less total airflow due tothe high temperature rise of a blow through direct fired system, and theflow path of the air may have less restriction or less static pressure.In a cooling mode, the amount of airflow must be increased above theheating mode level, and the air path is greatly restricted due to theadditional static pressure of the evaporator coil in the air streamcombined with higher velocities of air. The addition of a VFD allows fora combination of blow through direct heating, direct expansion (DX)cooling, and energy recovery ventilation (ERV) in one packaged unit withone supply air fan.

At least some of the example embodiments described herein may includevariable dampers, which allow the system to operate at other conditionsin cooling mode besides a 50%/50% outside/inside air mixture. Dependingon the enthalpy of the indoor and outdoor air (e.g., wet bulb and drybulb temperatures) and the indoor need for outside air, one or moredampers can vary to change the mixture from 50% outside air to 0%outside air. In situations where no additional outside air is needed tomeet the inside fresh air requirements, the system can run cooling using100% recirculated inside air, which may be more economical.

The system may include a sensor for sensing one or more operatingparameters of the system, including but not limited to, indoortemperature, outside temperature, dry bulb temperature, wet bulbtemperature, indoor air quality requirements ascertained by carbondioxide (CO₂) detection, etc. The sensed parameters may be monitored bya controller configured to operate the system at different air flowrates and/or different combination mixtures of indoor and outdoor air,to improve system performance (e.g., efficiency, air quality, etc.)based on the sensed parameters. For example, the controller may adjust aVFD of the blower motor to increase air speed based on detected indoorand outdoor temperatures, the controller may adjust variable dampers tochange the indoor/outdoor air mixture based on CO₂ detection, etc.

Exemplary embodiments of a direct fired heater disclosed herein may beconfigured such that they are associated with, include, allow, orprovide one or more (but not necessarily any or all) benefits including,providing blow through direct fired heating, direct expansion cooling,and/or energy recovery ventilation in a single packaged unit, preventingharmful combustion of refrigerant gasses, preventing harmful pressuresfrom developing in an evaporator coil, varying the amount of airdelivered through the system in different operating modes, varying themixture of inside and outside air to increase efficiency, increasedefficiency and/or economical savings for an HVAC system, etc.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms, and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail. In addition, advantages and improvements that maybe achieved with one or more exemplary embodiments of the presentdisclosure are provided for purpose of illustration only and do notlimit the scope of the present disclosure, as exemplary embodimentsdisclosed herein may provide all or none of the above mentionedadvantages and improvements and still fall within the scope of thepresent disclosure.

Specific dimensions, specific materials, and/or specific shapesdisclosed herein are example in nature and do not limit the scope of thepresent disclosure. The disclosure herein of particular values andparticular ranges of values for given parameters are not exclusive ofother values and ranges of values that may be useful in one or more ofthe examples disclosed herein. Moreover, it is envisioned that any twoparticular values for a specific parameter stated herein may define theendpoints of a range of values that may be suitable for the givenparameter (i.e., the disclosure of a first value and a second value fora given parameter can be interpreted as disclosing that any valuebetween the first and second values could also be employed for the givenparameter). For example, if Parameter X is exemplified herein to havevalue A and also exemplified to have value Z, it is envisioned thatparameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if parameter X is exemplified herein to have values in the range of1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may haveother ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10, 2-8, 2-3,3-10, and 3-9.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

The term “about” when applied to values indicates that the calculationor the measurement allows some slight imprecision in the value (withsome approach to exactness in the value; approximately or reasonablyclose to the value; nearly). If, for some reason, the imprecisionprovided by “about” is not otherwise understood in the art with thisordinary meaning, then “about” as used herein indicates at leastvariations that may arise from ordinary methods of measuring or usingsuch parameters. For example, the terms “generally,” “about,” and“substantially,” may be used herein to mean within manufacturingtolerances. Whether or not modified by the term “about,” the claimsinclude equivalents to the quantities.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements, intended orstated uses, or features of a particular embodiment are generally notlimited to that particular embodiment, but, where applicable, areinterchangeable and can be used in a selected embodiment, even if notspecifically shown or described. The same may also be varied in manyways. Such variations are not to be regarded as a departure from thedisclosure, and all such modifications are intended to be includedwithin the scope of the disclosure.

What is claimed is:
 1. A direct fired, blow through HVAC systemcomprising: a burner configured to heat air circulated across an openflame of the burner; a supply air blower disposed upstream of theburner; an evaporator coil configured to cool air through directexpansion, the evaporator coil disposed upstream of the burner; an airswitching box configured to selectively direct air through or around theevaporator coil; and a safety circuit configured to selectively turn offthe burner; wherein: the safety circuit is configured to, while thesystem is operating in a cooling and/or air-conditioning mode, ensurethe burner is off prior to any air being circulated across theevaporator coil; and the air switching box is configured to, while thesystem is operating in a heating mode and the burner is on, direct asupply of air around the evaporator coil to the burner so the supply ofair does not pass through the evaporator coil prior to entering theburner to ensure any potential leak of refrigerant will not pass throughan open flame of the burner.
 2. The system of claim 1, furthercomprising one or more inlets configured to provide air for circulationthrough the system.
 3. The system of claim 2, wherein the one or moreinlets are configured to provide: recirculated indoor air combined withoutdoor air while the system is in a cooling mode; and outdoor air onlywhile the system is in a heating mode.
 4. The system of claim 2, furthercomprising an energy recovery ventilation device configured to recoverenergy from conditioned air leaving an enclosed space and reintroducethe recovered energy into the air received at the one or more inlets forcirculation through the system.
 5. The system of claim 2, furthercomprising at least one sensor configured to detect an operatingparameter of the system.
 6. The system of claim 5, wherein the operatingparameter includes at least one of carbon dioxide, humidity, indoortemperature, outdoor temperature, and dry bulb temperature.
 7. Thesystem of claim 6, further comprising a controller coupled to one ormore components of the system and configured to control at least oneoperating characteristic of the system, based on the detected operatingparameter.
 8. The system of claim 7, wherein the at least one operatingcharacteristic includes one or more of an operating airflow, a gasmodulation, and a percentage of indoor and outdoor air combination. 9.The system of claim 8, wherein the one or more inlets comprise one ormore variable dampers, the variable dampers configured to adjust amixture of outdoor and indoor air supplied to the system.
 10. The systemof claim 8, wherein the supply air blower includes a variable frequencydrive configured to adjust an amount of airflow through the system basedon an operating mode of the system.
 11. The system of claim 1, furthercomprising: a compressor configured to pressurize a refrigerant; acondensing coil coupled to the compressor and configured to receive therefrigerant and convert the refrigerant from a gas to a liquid; one ormore condensing coil fans configured to circulate air across thecondensing coil; an expansion valve coupled to the condensing coil andconfigured to remove pressure from the refrigerant before providing therefrigerant to the evaporator coil; and a controller coupled to one ormore of the evaporator coil, compressor, condensing coil, one or morecondensing fans, and the expansion valve, the controller configured tooperate the HVAC in a cooling mode.
 12. The system of claim 11, furthercomprising one or more inlets configured to provide air for circulationthrough the system.
 13. The system of claim 12, further comprising anenergy recovery ventilation device configured to recover energy fromconditioned air leaving an enclosed space and reintroduce the recoveredenergy into the air received at the one or more inlets for circulationthrough the system.
 14. The system of claim 3, further comprising: acompressor configured to pressurize a refrigerant; a condensing coilcoupled to the compressor and configured to receive the refrigerant andconvert the refrigerant from a gas to a liquid; one or more condensingcoil fans configured to circulate air across the condensing coil; anexpansion valve coupled to the condensing coil and configured to removepressure from the refrigerant before providing the refrigerant to theevaporator coil; and a controller coupled to one or more of theevaporator coil, compressor, condensing coil, one or more condensingfans, and the expansion valve, the controller configured to operate theHVAC in a cooling mode.
 15. The system of claim 1, further comprising atleast one sensor configured to detect an operating parameter of thesystem, wherein the operating parameter includes at least one of carbondioxide, humidity, indoor temperature, outdoor temperature, and dry bulbtemperature.
 16. The system of claim 15, further comprising a controllercoupled to one or more components of the system and configured tocontrol at least one operating characteristic of the system based on thedetected operating parameter, wherein the operating characteristicincludes one or more of an operating air flow, a gas modulation, and apercentage of indoor and outdoor air combination.
 17. The system ofclaim 8, further comprising: a compressor configured to pressurize arefrigerant; a condensing coil coupled to the compressor and configuredto receive the pressurized refrigerant and convert the refrigerant froma gas to a liquid; one or more condensing coil fans configured tocirculate air across the condensing coil; an expansion valve coupled tothe condensing coil and configured to remove pressure from therefrigerant; an evaporator coil configured to receive the refrigerantfrom the expansion valve and cool air circulated through the systemthrough direct expansion; and a controller coupled to one or more of theevaporator coil, compressor, condensing coil, one or more condensingfans, and the expansion valve, the controller configured to operate thesystem in a cooling mode.